(MCT) CHICAGO—About 400,000 patients are transported in the U.S. each year by emergency medical helicopters, a vital and expanding service that has suffered more than 100 accidents and fatalities since 2003, including three more deaths on Monday in Illinois.

That accident record is unacceptable, the National Transportation Safety Board says.

The job of helicopter emergency medical crew member ranks as the highest-risk occupation in terms of fatality rates — more dangerous than deep-sea fisherman, logger and nine other hazardous jobs — according to a study a few years ago by Dr. Ira Blumen of the University of Chicago Hospitals.

The safety board has urged the Federal Aviation Administration in recent years to mandate safety improvements for the air-ambulance industry in place of voluntary compliance, a FAA strategy that critics say has not been effective enough. Broad inconsistencies still exist in pilot training, the degree of safety-related communications and navigation equipment aboard helicopter medical fleets and flight standards employed by independent operators that are hired mostly by hospitals, investigations into numerous accidents have shown.

But the fatal accident Monday night of an emergency medical chopper that crashed in a field about 30 miles south of Rockford appears to be linked to one of the most common causes of all aviation accidents — bad weather.

The last words over the communications radio from Andy Olesen, a Vietnam War-era helicopter pilot who commanded the twin-engine helicopter registered to Rockford Memorial Hospital, were that he and the two nurses on board were aborting the more than 50-mile flight to Mendota Community Hospital to pick up a patient and instead heading back to Rockford because of deteriorating weather conditions.

It’s uncertain what the weather was like aloft, at an estimated altitude between 3,000 and 4,000 feet. But the National Weather Service reported light snow and a temperature on the ground of 28 degrees Fahrenheit. A witness who said he heard the helicopter overhead said it was sleeting at the time.

Aviation experts said it’s not uncommon for a professional pilot, after conducting preflight preparations, to take off to determine the exact weather conditions, anticipating the possibility of turning back.

If the helicopter involved in Monday’s accident encountered freezing rain, the conditions may have disrupted the air flow over the foil-shaped rotor blades; caused ice to accumulate on the air frame, adding excess weight; and led to ice forming on the intake of the engines, reducing power, the experts said.

“Those three factors can come together to make it a bad day,” said Thomas Hagovsky, a helicopter expert who is an associate professor at Purdue University’s Department of Aviation Technology.

Emergency medical helicopter pilots routinely face challenging flight conditions, including flying at night, in a variety of weather and under tight deadlines to improve the survival chances of trauma victims or to transport organs for transplant to critically ill patients.

“It’s a difficult situation for the pilot because somebody is in trouble and they want to help,” Hagovsky said. “But the desire to participate should never get in the way of their decisions.”

The lead NTSB investigator assigned to Monday’s crash arrived at the scene Tuesday afternoon from Denver to begin an analysis that could take a year or longer to determine the probable cause of the accident, said Keith Holloway, a spokesman for the safety board.

The FAA is assisting in the investigation. Other parties to the investigation are likely to include the manufacturer of the approximately 25-year-old MBB BK 117 A-3 helicopter, a twin-engine medium utility rotorcraft that is often used for functions ranging from air ambulance to aerial crane work and law enforcement.

The safety board has investigated more than 230 crashes involving emergency medical service helicopters since 1983, according to a report issued last year by NTSB board member Robert Sumwalt. He noted that the board has made more than 50 recommendations to government and the air ambulance industry dating to 1988.

The industry has grown and experienced an increased number of accidents, according to a study released in 2007 by the U.S. Government Accountability Office that concluded better data collection on flight hours and other factors is needed for the FAA to provide more effective oversight.

“FAA’s main challenge in providing safety oversight for air ambulances is that its oversight approach is not geared toward air ambulance operations,” the GAO report said. “For example, FAA uses the same set of regulations to oversee air ambulance operations as it uses to oversee other air-taxi services” that operate on demand.

The NTSB has chronicled at least 110 helicopter emergency medical crashes, claiming more than 105 lives, from 2003 through July 24 of this year. The deadliest year on record was 2008, when 29 people died in 13 accidents, safety board records show.

In 2007, the FAA announced that a review of air ambulance helicopter accidents from 1998 through 2004 showed a need for more detailed flight planning by operators of the service. The FAA urged operators to take advantage of a digital data program containing weather information and forecasts, as an added tool to make better decisions about whether to fly on medical missions.

But Sumwalt’s report said the accident record involving emergency medical helicopters is unacceptable. “Not all air ambulance operators are created equally from a safety perspective,” he said.

In 2009, the safety board issued 21 safety recommendations to the FAA and industry, covering pilot training, aircraft equipment and other criteria. They included the use of flight simulators and other training devices such as night-vision imaging systems to ensure pilot proficiency for inadvertent flight into clouds or other low-visibility conditions. The agency also required autopilot technology on helicopters if a second pilot is not part of the crew.

Few of the recommendations have been officially adopted, safety board officials have said, expressing frustration with the slow pace of the FAA rule-making process.

But some air ambulance companies have voluntarily upgraded their equipment and pilot training because they are committed to safety, according to the Association of Air Medical Services, an industry trade group.